1. Field of the Invention
This invention relates to semiconductor fabrication, and more particularly to a method for fabricating a phase shift mask (PSM) by automatically forming a phase shifting layer and a shielding layer, using an original program designer file.
2. Description of Related Art
Currently, photolithography plays an essential role in semiconductor fabrication. For example, processes for patterning a structure or doping a region in a wafer always need several photolithography processes. In a photolithography process, exposure resolution and depth of focus (DOF) are two important parameters that determine photolithography quality. As the integration of semiconductor devices increases, a high pattern exposure resolution becomes critical. One proposed solution to obtaining a high pattern exposure resolution would use a light source with a shorter wavelength, such as a deep ultraviolet light with a wavelength of 2480 angstroms produced by a Kr laser. However, even though a light source with shorter wavelength can increase the pattern exposure resolution the DOF deteriorates. Recently, another solution is proposed that uses a phase shifting, mask (PSM) in photolithography technology so as to obtain a high pattern resolution.
The theory of a PSM is to use a shifter layer formed on a conventional photomask. The shifter layer has a light inversion property that inverts the phase of light passing through the shifting layer. When the PSM is exposed to a light source, the light passing the shifting layer has an inverted wave phase and can interfere with a normal light, having not passed through the shifter layer. Since the amplitude of the inverted wave phase is negative an amplitude subtraction occurs around the interface between the normal light and the inverted wave light, resulting in a zero amplitude. When a wafer is exposed by light that has passed through the PSM, a light intensity distribution, which is the square of the amplitude distribution, with better resolution appears on the wafer. The light intensity distribution actually forms a desired pattern with a better pattern resolution. Using the PSM to obtain a better pattern resolution does not need a new light source with a short wavelength. This is the advantage of the PSM.
FIGS. 1A-1C are schematic. cross-sectional views of some conventional PSMs. In FIG. 1A, a general PSM conventionally has a shielding layer 12 including chromium and a phase shifting layer 14 formed on a transparent quartz substrate 10. The phase shifting layer 14 fully fills some of the regions of the quartz substrate 10 exposed by the shielding layer 12. In FIG. 1B, a rim PSM conventionally has a shielding layer 12 on the quartz substrate 10. and a larger shifting layer 16 covering the shielding layer 12. In FIG. 1C, an alternating PSM conventionally has a phase shifting layer 18 on the quartz substrate 10 with some regular openings, and a shielding layer 12 alternatingly fills the openings of the shifting layer 18. The shifting layers 14, 16, and 18 include MoSi.sub.Z O.sub.X N.sub.Y, or SiO.sub.X N.sub.Y can shift the light wave phase by 180.degree..
FIG. 2 is a light amplitude distribution on the PSM of FIG. 1C. The horizontal axis is the location across the alternating PSM. Referring to FIG. 1C and FIG. 2, the negative amplitude comes from the phase shifting layer 18, which shifts the light wave phase by 180.degree.. The positive amplitude results from the exposed portion of the quartz substrate 10. The actual amplitude is the summation of the positive amplitude and the negative amplitude. A zero amplitude can occur at the interface between the positive amplitude and the negative amplitude. After taking the square of the amplitude distribution, a light intensity with a better pattern resolution is obtained. The actual exposed light on the wafer is proportional to the intensity.
However, all of above three types of PSM need at least two patterning processes. One is the shielding layer and another one is the phase shifting layer. Since the desired patterns are usually designed by engineers, it is easy to make an accidental design error, and time is consumed.